Apostles are high ranking servants of The Archangel. There are a total of twelve of them at any given point in time, and they all are loosely connected, with a low level of communication and co-work. To become an Apostle, one has to find "favor" with the Archangel, but the way to find favor is unknown. They're placed above the cultists in what chain of command exists, and co-work with the Prophet.
They wear gas masks and black hoodies with the Twin Triangles on them in an attempt to emulate the characteristics of the Archangel. They act like most servants do; killing, intimidating, etc, but with a higher enphasis on destruction. In addition, they are given a small power boost to be able to do what he commands.
Apostles first appeared in Hidden Triumphant, where Paradoxical Machinations eventually became one. Another Apostle appears as an antagonist in The Thirteenth Apostle (the title refers to the Apostles of Jesus, not the Archangel). This Apostle goes by the name "Ophilim" and leads the Timberwolves in New Rossfield.